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lAüfotr’A, byAsr. Newton.n,-9&, f, 6 o?i.

^4 CiU-Dioptri-.-tl Ttlefiope ;fc_j M. Casle-grain. n. 8 z,f ~. 405 6.

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ConßAtr’J by

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excefs ol Denilty, the Light, which falls in or invifibly near the jultPoint, may I conccive, ltrike the Schsoriüm so vigorously, that the Im-prcls of the weak Light, which Errs round about it, {hall, in comparilbn,110t be strong enough to be animadvertcd, or to caule any more sensible (Kon-fusion in the Picture than is found by Experience. But if this latisfic not,As. may try, if he plcaso, how distinct the Picture will appear, when all theLens is cover’d excepti ng_a little hole nex't its Edge on one side only : Andit in this caso he plcale. tQ mealüre the breadth of die. Colours thus made atthe Edge of the Sun’s Picture, he will perhaps fjnd.it. to approach ncarcr tomy Proportion than he expects.

4. I am lätisfied with the manner, whcreby Mr. Newton reconciles the eftectof Convex Glaifes with bis Thcory, but then he is also to acknowledgc,that this Aberration of the Rays-is-not so dilädvantageous to Optick-Glalfesas he leems to have been willing to makc us believe. His Invcntion is verygood j but the defect of the Metal leems to rendet >t as impollible to exe-cute, as tHe difficulty of. the Form obstructs the uso of the .Hyperbole cfM, Des C artes. ■

1 t .M. N .pleales to compute the Errors of a Glaß and. Speculum

that collect Rays at cqual Distances, he will find how much he is mistaken ;and that I have not been extravagant, as he imagines, in preferring. Re-flexions.. And as.for what helays of the difficulty of the Praxis, 1 know icis very difficult, and by thole ways which he attempted it 1 believe it impracti;cable. But there is a way instnuated above, by which it is not imprpblable butthat as much may be done in large Tclelcopes, as I have tbetehy do.ne so lliortones, but yei not without more than ordinar.y Diligence and Curiosity

- XIX. 1.. M. Caffegrain has comraunicated the FigurOiof.a Telclcope, al->most like that of Mr. Newton.

AB CD, ka strong Tube, in the bottom of wiiich there is a greatConcave Speculum C D, pierced in the middle E- .

F is a. Convex Speculum, so dilpoled, as to its Convexity, that it ReHectsthe Species, which it reoeives from the great Speculum, towards the Hole E,where is an Eye Glals, which one looketh through.

The Advantage which I find in this Instrument above that of Mr. Piewton,is sirst, That the Mouth or. Aperture, AB-of the.T-ybe .may be ,of whatbignels you pleafc ; and conlequently you may have many more Rays uponthe Concave Speculum,, than upon that, of which you have given, us the,Delcriptign. i. The Reflexion of the Rays will be very natural, sn;ce it willbc made upon the Axis it solf, and therefore more Vidid. 3. The Vision ofit,-will be so much the more pleasing, in that you fhall not be incommodedby the great Light, by reason of the bottom; CD, which hideth the whole,Face. Besidesj -.you’ll. have lesi difficulty - in .disooverjng the Objects than . in,-that. of Mr. Newton.

a. Whenlfirst applied my solf to try the Effects of Reflex-ions, Mr. Crc—gor/ s Optica Prcn/ota,. (Printed jn the.Ycir 1 66g.) being fallen into my hand-V,wfere.there is an Instrument; (desoribcd />. 94.) like that oi Mr. Ga/jcgraws,,... w:tl